1. Field of the Invention
The subject invention is directed to a method for etching silicon to achieve desirable surface characteristics, and more particularly, to a method of using additives in the caustic etching process of silicon to, among other things, reduce the relative roughness of the silicon surface.
2. Background of the Related Art
Wet etching is one of the processes used to manufacture silicon wafers. Lapped wafers are subjected to wet etching to clean post-lapped surfaces and to eliminate the subsurface damage by removing a layer of silicon.
Although many different etching formulations are known and used, they can be grouped into two basic types. The first type is referred to as acid etching, which involves the use of hydrofluoric and nitric acids with water and/or acetic acid as diluents. The second type is referred to as caustic etching, which involves the use of alkali metal hydroxides.
Due to the nature of the chemical reactions between the active reagents and silicon surface, quite different wafers are produced depending on which of these two etchants are used. Despite the variety of formulations with different properties, the isotropic acid process is generally said to be diffusion controlled, yielding relatively smooth surface that are free of features (i.e., unfaceted). However, there is a relatively poor uniformity of silicon removal across the wafer, which results in significant total thickness variation (TTV).
The anisotropic caustic process is said to be reaction controlled and even though it is characterized by a good uniformity of silicon removal (i.e., a low TTV), it suffers from a surface topography generated (i.e., a faceted surface) and much higher roughness.
The higher roughness of wafers obtained from caustic etching results from the characteristic features (or facets) that form on a surface, such as squares (or pillows) for a (100) wafer and triangles for a (111) wafer. The size of these features increases with the amount of silicon etched and can be significant. For example, the squares on a surface of a (100) wafer with 25 μm of silicon etched (12.5 μm on each side) can have edges of ca. 20 μm.
It is highly desirable to obtain silicon wafers characterized by a good uniformity across the wafer and a surface with low roughness. One possibility is to find additives to the caustic process that would affect the etching and produce smoother surfaces. Technical literature, as well as industrial R&D practice, indicate that there have been numerous attempts to accomplish this goal. Isopropanol was the most often mentioned additive, which, according to these sources, was seemingly found to reduce roughness. However, the improvements were not significant. Furthermore, the practical difficulties associated with using isopropanol at elevated temperatures rendered using isopropanol a nonviable option.
Other additives were reported to change the appearance of the etched surface, but only to a small extent. More specifically, in the case of (100) wafers, some rounding of the squares was noticed, without changing the size and orientation of these features. However, none of the additives provided a desirable level of change to the faceted appearance and roughness of the silicon surface produced from the caustic process. In the quest to produce flatter and smoother wafers, silicon wafer manufacturers continually consider modifications to the acid process that would result in improved wafer geometry, but have had problems developing a successful, practical modification.
An alternative solution to this problem would be to improve the caustic process in a way that would result in wafers with lower roughness. What is needed is an additive that would reduce the size of the features on the wafer surface, since surface roughness is proportional to the size of features on the surface. Thus, the present invention advantageously provides a solution, which is practical and successful, that employs an additive and method of using the additive in conjunction with a caustic etching process to produce a silicon wafer having improved surface characteristics, such as reduced surface features, among other things.